I can host and load a local server myself but everyone else that joins gets a message Map String differ or a error message that the map is different

It means that one of your map files is different than the ones who are trying to connect. Verify your game cache and it should clear that message up.

@OP: I have had no problem hosting local games recently. Hosted one today actually with three other friends and it worked great. I have no addons though so that might be what is causing it.

There aren't any problems with the map, since I don't really have anything in the files that changes them in any way.

It has done this once in the past not very long ago for a while since I got the game, then it seemed to fix itself with an update (not the latest one).

One of my main focuses are the soundfiles I've edited. Yet, I'm not entirely sure that's the case, because they're not VPK addons, they're properly named replacements of the original files. For example: when I play on a dedicated server mode, the edited soundfiles are present, but not the VPK addons.

But maybe it is the addons that are causing the problem, and turning some off might help...